I love my Boye set! 13 needle sizes, ranging from US size 2 to 15! Nice smooth needles that are a pleasure to knit on. Very compact carrying case, measuring at only 7" x 7-1/2" x 3/4". The only thing it needs, in my opinion, is a different rubber grip to better tighten the needles. I use a circle cut from a pair of dish-washing gloves; works like a charm, I never ever have needles unscrewing on me. I can't speak to the Denise set, because I've never used it.
I'm not sure about other sets, but I don't think
any will give smaller than 20". The reason, is because you can't have a short cable, without also shortening the straight-needle parts. If you look at 16" cable needles, their straight-needle parts (the ends that you hold onto and knit with) are significantly shorter than those on larger cable needles. Many people find them harder to knit with, because there's less needle to hold onto. So, it's understandable that the interchangeable kits make the needle sizes big enough to hold onto, and, consequently, too big to make a 16" set out of.
The kit-owner's solution to doing smaller diameter knitting is (listed in order of affordability)...
1) Use the Magic Loop method to knit smaller items. Not the most fluid method of knitting, in my mind, but certainly economical, and always good to know. With a kit, you use a double-length cable (two cables attached together), for an extra-long cable length.
2) Buy DPN's in your favorite sizes.
3) Buy 16" circulars, in your favorite sizes. Then you can do 16" items like hats, and you'll have the second cable needle that can be used for doing small-diameter knitting on two cable needles. If you don't mind knitting with the smaller needle ends that are on a 16", then this option makes a lot of sense!
4)Own two kits! If you like knitting on two-circulars, which is a nice way to knit, then this is a great solution for your smaller diameter knitting.
I think my preferences are largely based on habit. I think any of the above solutions could work great for you.